Bearing with magnetic fluid seal

ABSTRACT

Provided is a bearing with a magnetic fluid seal reliably maintaining sealing of a rolling element section in the bearing and having a structure ensuring high productivity. In a bearing with a magnetic fluid seal according to the present invention, a plurality of rolling elements  7  are disposed between an inner ring and an outer ring, and a ring-shaped magnet is arranged at an opening side of the inner and outer rings so as to retain magnetic fluid, so that the plurality of rolling elements are sealed. The magnet is magnetized so that magnetic poles are oriented in an axial direction. A ring-shaped pole plate is arranged to be in contact with an outer side surface of the magnet in the axial direction. Outer-ring-side magnetic fluid is retained between the outer ring and the magnet, and inner-ring-side magnetic fluid is retained between the inner ring and the pole plate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/342,747, filed Mar. 4, 2014, which is a U.S. National Stage ofPCT/JP2013/061269, filed Apr. 16, 2013, which in turn claims priority toJapanese Patent Application No. 2012-100551, filed Apr. 26, 2012, theentire contents of all applications are incorporated herein by referencein their entireties.

TECHNICAL FIELD

The present invention relates to a bearing with a magnetic fluid sealwhich supports a rotating shaft in a rotatable manner in various powertransmission mechanisms, the magnetic fluid seal preventing foreignmatter, such as dust and moisture, from intruding into an inner regionof the bearing.

BACKGROUND ART

In general, rotating shafts installed in various drive powertransmission mechanisms are rotatably supported by bearings. In thiscase, so-called ball bearings are often used. Ball bearings include aplurality of rolling elements (rolling members) arranged in acircumferential direction in a space between an inner ring and an outerring. The rotational performance of rotating shafts can be increased byusing bearings of this type.

This type of bearing is used as means for supporting a rotating shaft ina drive power transmission mechanism for various drive devices. Somedrive devices are required to prevent foreign matter, such as dust andmoisture, from intruding into an inner region thereof through a bearingsection. If foreign matter intrudes into the bearing itself, there willbe a problem that the rotational performance will be degraded orabnormal noise will be generated. To solve this problem, a seal membermade of an elastic material may be arranged on the outer periphery ofthe rotating shaft in the vicinity of the bearing, so that the bearingsection is waterproof and dustproof. However, in this case, therotational performance of the rotating shaft is degraded due to theinfluence of a contact pressure applied by the sealing member made of anelastic material.

Bearings having magnetic fluid seal mechanisms in which magnetic fluidis used (hereinafter referred to as bearings with magnetic fluid seals)are known structures that prevent foreign matter from intruding into thebearing section without reducing the rotational performance of therotating shaft. For example, PTL 1 discloses a ball bearing in whichrolling elements are retained between an outer ring and an inner ringand in which a magnetic member is interposed between the outer ring andthe inner ring that rotate relative to each other, the magnetic memberbeing fixed at one side thereof and magnetic fluid being disposed in aseal gap provided at the other side of the magnetic member. Morespecifically, the magnetic member is disposed between the inner ring andthe outer ring so as to block the rolling elements, the magnetic memberbeing fixed at one side thereof and the magnetic fluid being disposed inthe seal gap provided at the other side of the magnetic member.Accordingly, the rolling elements are tightly sealed and intrusion offoreign matter into a rolling element section, which affects therotational performance, is prevented.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 57-33222

SUMMARY Technical Problem

The above-described bearing with the magnetic fluid seal disclosed inPTL 1 provides a sufficient sealing effect for dust and liquid having arelatively high viscosity. However, there is a possibility that thesealing effect will not be sufficient for liquid having a low viscosity.More specifically, the dimensional accuracy of the magnetic member islower than those of components included in the bearing, and thereforethe magnetic member easily allows liquid to intrude into the bearing atthe fixed side thereof. The lower the velocity of the liquid, the moreeasily the liquid intrudes into the rolling element section (inparticular, intrusion of seawater leads to a degradation of rotationalperformance since the seawater that has intruded dries and forms saltcrystals). In this case, to reliably seal the bearing, it is necessaryto accurately control the dimensions of the magnetic member or installadditional sealing components. As a result, productivity will bereduced.

The present invention has been made in light of the above-describedproblems, and an object of the present invention is to provide a bearingwith a magnetic fluid seal which reliably maintains the state in which arolling element section in the bearing is sealed and which has astructure that ensures high productivity.

Solution to Problem

To achieve the above-described object, according to the presentinvention, a bearing with a magnetic fluid seal, wherein a plurality ofrolling elements are interposed between an inner ring and an outer ring,and a ring-shaped magnet is arranged at an opening side of the inner andouter rings so as to retain magnetic fluid, so that the plurality ofrolling elements are sealed, is characterized in that the ring-shapedmagnet is magnetized so that magnetic poles are oriented in an axialdirection, and the bearing includes a ring-shaped pole plate arranged soas to be in contact with an outer side surface of the ring-shaped magnetin the axial direction; outer-ring-side magnetic fluid retained betweenthe outer ring and the ring-shaped pole plate and/or between the outerring and the ring-shaped magnet; and inner-ring-side magnetic fluidretained between the inner ring and the ring-shaped pole plate and/orbetween the inner ring and the ring-shaped magnet.

In the above-described structure, the ring-shaped magnet is magnetizedso that the magnetic poles are oriented in the axial direction, and thering-shaped pole plate is arranged so as to be in contact with the outerside surface of the ring-shaped magnet in the axial direction.Therefore, the magnetic fluid (outer-ring-side magnetic fluid) can beretained between the outer ring and the ring-shaped pole plate and/orbetween the outer ring and the ring-shaped magnet; and the magneticfluid (inner-ring-side magnetic fluid) can be retained between the innerring and the ring-shaped pole plate and/or between the inner ring andthe ring-shaped magnet. In other words, the magnetic fluid may beretained in both a gap on the inner peripheral surface of the outer ringand a gap on the outer peripheral surface of the inner ring. Therefore,sufficient sealing effect can be achieved for the rolling elements evenwhen the dimensional accuracy of the ring-shaped magnet is low, and itis not necessary to accurately control the dimensions of the ring-shapedmagnet. As a result, the assembly can be facilitated and productivitycan be increased.

The magnetic fluid retained on the inner peripheral surface of the outerring and the outer peripheral surface of the inner ring may either beprovided at only one of the openings at the ends of the bearing or beprovided at both of the openings.

Advantageous Effects

According to the present invention, a bearing with a magnetic fluid sealwhich reliably maintains the state in which a rolling element section inthe bearing is sealed and which has a structure that ensures highproductivity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of a bearing with a magnetic fluid sealaccording to a first embodiment of the present invention taken in anaxial direction.

FIG. 2 is an enlarged view of a relevant part in FIG. 1.

FIG. 3 is a diagram illustrating a modification of the first embodiment.

FIG. 4 is an enlarged sectional view of a relevant part of a bearingwith a magnetic fluid seal according to a second embodiment of thepresent invention taken in an axial direction.

FIG. 5 is a diagram illustrating a modification of the secondembodiment.

FIG. 6 is a sectional view of a bearing with a magnetic fluid sealaccording to a third embodiment of the present invention taken in anaxial direction.

DESCRIPTION OF EMBODIMENTS

Bearings with magnetic fluid seals according to embodiments of thepresent invention will now be described with reference to the drawings.

FIGS. 1 and 2 illustrate a bearing with a magnetic fluid seal accordingto the first embodiment of the present invention. FIG. 1 is a sectionalview taken in an axial direction, and FIG. 2 is enlarged view of arelevant part in FIG. 1.

The bearing with a magnetic fluid seal (hereinafter referred to also asa bearing) 1 according to this embodiment may include a cylindricalinner ring 3, a cylindrical outer ring 5 that may surround the innerring 3, and a plurality of rolling elements (rolling members) 7 that maybe interposed between the inner ring 3 and the outer ring 5. The rollingelements 7 may be retained by a retainer (retaining member) 8, which mayextend in a circumferential direction, and the inner ring 3 and theouter ring 5 may be rotatable relative to each other.

The inner ring 3, the outer ring 5, and the rolling elements 7 may bemade of a magnetic material, such as a chromium stainless steel(SUS440C). The retainer 8 may be made of a material having a highcorrosion resistance and a high heat resistance, such as a stainlesssteel (SUS304). It is not necessary that the rolling elements 7 bemagnetic members. In this embodiment, the outer ring 5 may be formedsuch that an exposed end surface 5 a thereof is flush with (orsubstantially flush with) an exposed end surface 3 a of the inner ring3. However, as in a third embodiment, which will be described below, theouter ring 5 may be longer than the inner ring 3 in the axial direction(the outer ring 5 may include an elongate cylindrical portion thatprojects from the inner ring 3 in the axial direction). Alternatively,the inner ring 3 may be longer than the outer ring 5 in the axialdirection.

Magnetic fluid seals 10, which will be described in detail below, may beprovided at opening sides of the inner ring 3 and the outer ring 5. Inthis embodiment, magnetic fluid seals having the same structure may beprovided at the openings at both sides of the inner ring 3 and the outerring 5. Therefore, in the following description, the structure at onlyone side (left side in FIG. 1) will be described.

The magnetic fluid seal 10 may include a ring-shaped magnet (hereinafterreferred to as a magnet) 12, a ring-shaped pole plate (hereinafterreferred to as a pole plate) 14 arranged so as to be in contact with anouter side surface of the magnet 12 in the axial direction, and magneticfluid (outer-ring-side magnetic fluid 15 a and inner-ring-side magneticfluid 15 b) retained by magnetic circuits formed by the magnet 12. Thesecomponents may provide a sealing function for preventing dust, moisture,etc., from intruding into a region around the rolling elements 7.

The magnet 12 may be a permanent magnet having a high magnetic fluxdensity and a strong magnetic force; for example, a neodymium magnetformed by sintering may be used. As illustrated in FIG. 2, the magnet 12may be magnetized in advance so that magnetic poles (S pole and N pole)are oriented in the axial direction (direction of central axis X of thebearing). The pole plate 14 may be disposed so as to be in contact withthe outer side surface of the magnet 12 in the axial direction. The poleplate 14 and the magnet 12 may have substantially the same shape, andthe pole plate 14 may be made of a magnetic material, such as a chromiumstainless steel (SUS440C).

In this embodiment, the magnet 12 and the pole plate 14 may be bondedtogether in advance; however it is not necessary to bond them togetherin advance. When the magnet 12 and the pole plate 14 are bonded togetherin advance, positioning and centering of the magnet 12 can befacilitated. In addition, since the magnet 12 and the pole plate 14 areunitized, the assembly process, which will be described below, can alsobe facilitated.

The outer-ring-side magnetic fluid 15 a and the inner-ring-side magneticfluid 15 b may be formed by dispersing fine magnetic particles composedof, for example, Fe3O4, into a surface-active agent and a base oil. Theouter-ring-side magnetic fluid 15 a and the inner-ring-side magneticfluid 15 b may be viscous and have a characteristic such that they reactto a magnet when the magnet is brought close thereto. Therefore, theouter-ring-side magnetic fluid 15 a and the inner-ring-side magneticfluid 15 b may be reliably retained at predetermined positions bymagnetic circuits formed by the magnet 12 and the inner ring 3, theouter ring 5, and the pole plate 14, which may be formed of a magneticmaterial.

A step 5 b may be formed on an inner surface of the outer ring 5 at aposition closer to the rolling elements than the magnet 12. Owing tothis step 5 c, the outer ring 5 may include a thin portion 5A at theopening side and a thick portion 5B at the rotating-element side, andthe distance between the inner and outer rings may be greater at theouter side than at the inner side in the axial direction. The step 5 bmay be formed so that a gap for retaining the magnetic fluid (step gap)is provided. In this embodiment, the step 5 b may be formed so as tohave a perpendicular surface 5 c that is perpendicular to the axialdirection (when the perpendicular surface is formed, the magnet 12 maybe attached to the perpendicular surface by attraction so that themagnet 12 is positioned and secured, as described below). The step isnot limited to that having a perpendicular surface as in thisembodiment, and may instead have a staircase shape or an oblique shape(oblique surface) as long as the magnetic fluid can be reliably retainedbetween the step and the magnet 12. In this case, when an obliquesurface is formed, the magnet 12 can be positioned and the magneticfluid can be retained.

The pole plate 14 may be formed such that the outer diameter thereof isslightly larger than the diameter of the inner peripheral surface of theouter ring 5 (inner peripheral surface of the thin portion 5A), and maybe press-fitted to the outer ring 5 together with the magnet 12 bondedthereto at the opening side of the outer ring 5. The size of the poleplate 14 having the magnet 12 bonded thereto may be such that apredetermined gap G is formed between the pole plate 14 and the outerperipheral surface of the inner ring 3 when the pole plate 14 ispress-fitted to the outer ring 5. The lengths of the magnet 12 and thepole plate 14 in the axial direction may be such that a gap G1 is formedon the perpendicular surface 5 c of the step 5 b when the magnet 12 andthe pole plate 14, which are bonded together, are press-fitted.

When the pole plate 14 having the magnet 12 bonded thereto, the magnet12 being magnetized so that the magnetic poles are oriented in the axialdirection, is press-fitted to the outer ring 5 as described above,magnetic fluxes (magnetic circuits 3M and 5M) symmetrical in the axialdirection may be formed at the inner-ring-3 side and the outer-ring-5side, as illustrated in the figure. Therefore, the inner-ring-sidemagnetic fluid 15 b may be retained in the above-described gap G betweenthe pole plate 14 and the inner ring 3, and the outer-ring-side magneticfluid 15 a may be retained in the gap G1 between the magnet 12 and theouter ring 5. More specifically, when the magnetic fluid is injectedinto the gap G with an injection device, such as a syringe, the magneticfluid may be retained in the gap G by the magnetic circuit 3M. Inaddition, the magnetic fluid may flow to the gap G1, and be retainedalso in the gap G1 by the magnetic circuit 5M formed at the outer-ringside.

With the bearing 1 having the above-described structure, the sealingeffect can be provided also at the fixed side at which the magnet 12 andthe pole plate 14 are fixed (on the inner peripheral surface of theouter ring 5 in this embodiment). Therefore, intrusion of moisturehaving a low viscosity or dust toward the rolling elements 7 can bereliably prevented at the fixed side. In a bearing with a magnetic fluidseal according to the related art, the necessity of a seal at the sideat which a magnet and a pole plate are fixed is not considered, and thesealing effect for the rolling elements is not sufficient. In contrast,in this embodiment, a seal may be formed not only by the inner-ring-sidemagnetic fluid 15 b but also by the outer-ring-side magnetic fluid 15 aat the fixed side. Therefore, a sufficient sealing effect can beprovided.

The seals may be formed simply by magnetizing the magnet 12, which maybe a single member, so that the magnetic poles are oriented in the axialdirection and arranging the magnet 12 so as to be in contact with thepole plate 14; therefore, the number of components may be small. Inaddition, it is not necessary that the dimensions of the magnet 12 beaccurately controlled. As a result, the assembly can be facilitated andthe cost can be reduced. In other words, even when a magnet whosedimensional accuracy is lower than those of other members is used, asufficient sealing effect can be provided.

Furthermore, the magnet 12, which may retain the magnetic fluid at boththe inner-ring side and the outer-ring side, may be formed as a singlemember, and the magnetic fluid seals at the inner-ring side and theouter-ring side can be formed at the same time by injecting the magneticfluid at a single position. As a result, the work efficiency isincreased.

In addition, in this embodiment, since the step 5 b is formed on theouter ring 5, the space (step gap) in which the magnetic fluid can bereliably retained can be formed by using the step, and the sealingeffect can be easily increased. Although the outer-ring-side magneticfluid 15 a is retained in the gap G1 in FIG. 2, the outer-ring-sidemagnetic fluid 15 a may also be provided in a gap between the outerperipheral surface of the magnet 12 and the inner peripheral surface ofthe outer ring 5 and a small gap between the pole plate 14 and the innerperipheral surface of the outer ring 5. Thus, a sufficient sealingfunction can be provided at the outer-ring side.

FIG. 3 is a diagram illustrating a modification of the above-describedembodiment.

In the embodiment illustrated in FIGS. 1 and 2, the pole plate 14 has aconstant thickness in the radial direction. However, as illustrated inFIG. 3, the pole plate 14 is preferably tapered so that the thicknessthereof gradually decreases toward a portion that retains the magneticfluid (radially inner portion in this embodiment) (thin portion isdenoted by reference symbol 14A).

With this structure, the magnetic fluid 15 b does not spill outward(beyond the exposed end surfaces 5 a and 3 a of the outer and innerrings) in the axial direction. Therefore, the magnetic fluid may beprevented from being wiped off during the assembly, and the injectionprocess can be reliably performed.

FIG. 4 illustrates a second embodiment of the present invention.

In this embodiment, the dimension of the pole plate 14 in the radialdirection is set so that a certain clearance (gap G2) is providedbetween the pole plate 14 and the inner peripheral surface of the outerring 5.

Therefore, when the pole plate 14 having the magnet 12 bonded thereto(it is not necessary that the magnet 12 be bonded to the pole plate 14)is simply inserted from the opening side of the inner and outer rings,the magnet 12 may come into contact with the perpendicular surface 5 cof the step due to a magnetic attraction force, and thereby bepositioned and secured. In this state, when the magnetic fluid isinjected into the gap G and the gap G2 with an injection device, such asa syringe, the magnetic fluid is retained in the gap G (between theinner ring 3 and the pole plate 14) by the magnetic circuit 3M and inthe gap G2 (between the outer ring 5 and the pole plate 14) by themagnetic circuit 5M. The magnetic fluid that fills the gap G may flowtoward the step and be retained also in a step portion between themagnet 12 and the outer ring 5 (region between the magnet 12 and theouter ring 5). Therefore, the sealing effect at the outer-ring side maybe increased.

With this structure, the pole plate 14 having the magnet 12 bondedthereto can be easily assembled. In addition, in the case where theopenings at both sides of the bearing are to be sealed, the orientationsof the magnets can be easily controlled. In addition, since the poleplate 14 is formed so that the gap G2 is provided, the outer ring 3 doesnot receive a deformation load during the assembly, and the rotationalperformance of the bearing is not degraded. In consideration of the easeof assembly and the sealing effect, the gap G2 may be set in the rangeof 10 to 500 μm, preferably in the range of 20 to 200 μM.

FIG. 5 is a diagram illustrating a modification of the secondembodiment.

In this modification, a step 3 b may be formed on an inner ring, and aperpendicular surface 3 c that is perpendicular to an axial directionmay be formed on the step 3 b. A magnet 12 and a pole plate 14 similarto those in the structure illustrated in FIG. 4 may be disposed on theinner ring (the structure may be symmetrical to that illustrated in FIG.4).

The bearing having this structure may be suitable for a case in whichthe outer ring is attached to a rotating member.

Also in the structures illustrated in FIGS. 1 to 3, the magnet 12 andthe pole plate 14 may be attached to the inner ring as in the structureillustrated in FIG. 5.

FIG. 6 is a diagram illustrating a third embodiment of the presentinvention.

In this embodiment, an outer ring 5 may be formed so as to be longerthan an inner ring 3 in an axial direction, and may include an elongatecylindrical portion 5D that projects from an exposed end surface 3 a ofthe inner ring 3. A magnetic fluid seal 10 having a structure similar tothe above-described structure may be provided on the elongatecylindrical portion 5D.

In this case, the length of the elongate cylindrical portion 5D in theaxial direction may be set so that a gap G3 is provided between a magnet12 and the exposed end surface 3 a of the inner ring 3 in the state inwhich the magnet 12 is attached to a perpendicular surface 5 c of a step5 b on the outer ring by attraction and is thereby positioned andsecured. In the structure of this embodiment, the dimension H of themagnet 12 in the radial direction may be larger than those in theabove-described embodiments and modifications. Therefore, the magneticforce can be increased and the sealing effect (ability to retain themagnetic fluid) can be enhanced. In addition, since inner-ring-sidemagnetic fluid 15 b may be disposed in an inner region in the axialdirection so as not to be exposed to the outside, the inner-ring-sidemagnetic fluid 15 b may be prevented from being wiped off during theassembly, and an injection process can be reliably performed. Althoughthe outer ring 5 has the elongate cylindrical portion in thisembodiment, the inner ring 3 may instead have an elongate cylindricalportion.

In the structures of the above-described embodiments and modifications,the surfaces of the inner ring 3 and the outer ring 5 are preferablytreated with electrolytic chromic acid. When the treatment withelectrolytic chromic acid is performed, the occurrence of cracks andtears in the surfaces due to rust or corrosion can be prevented, andintrusion of dust and foreign matter into an inner region can bereliably prevented.

In addition, in the above-described structures, a ring-shaped shield(sealing cover) may be press-fitted to the outer surface of the poleplate 14, which is disposed at the opening side, in the axial directionfrom an outer position in the axial direction. The shield may be formedof a material having a high corrosion resistance and a high heatresistance, such as a stainless steel (SUS304) or a resin. When theshield is provided, intrusion of foreign matter can be more effectivelyprevented. In addition, magnetic matter such as iron sand (foreignmatter) can be effectively prevented from adhering to the magnet 12.

In addition, in the above-described structures, a thin washer or apositioning spacer member may be disposed between the magnet 12 and theouter ring 5 (or the inner ring 3). When such a washer or a spacer isprovided, dimensional control can be simplified and ease of assembly canbe further improved. The washer or the spacer is preferably formed of amagnetic member so that stable magnetic circuits can be formed.

EXAMPLES

The bearings with the magnetic fluid seals having the above-describedstructures may be attached to rotating shaft portions of various devicesthat are required to be dustproof and waterproof. In particular, severeconditions are expected in an environment where salt (seawater) exists.More specifically, seawater easily intrudes through small gaps since ithas a low velocity, and when the seawater that has intruded dries, itleaves salt crystals. If these crystals adhere to the rolling elements,the rotational performance will be greatly degraded.

When the bearings according to the above-described embodiments areattached to driving shaft portions of power transmission units includedin various fishing reels used for fishing by the seashore or on the sea,the driving shaft sections can be stably supported for a long period oftime.

Table 1 shows the result of a test in which the bearings according tothe above-described embodiments and bearings having the structuresaccording to the related art were attached to rotating shaft portionsthat are rotated by handles of spinning reels, and in which therotational performances of the bearings (degrees of smoothness theoperator feels when rotating the handles) were evaluated after immersingthem in salt water.

In Table 1 shown below, Related Art Example 1 is an example in which abearing on a handle shaft (rotating shaft) is sealed with a commonlyknown rubber packing material. Related Art Example 2 is an example inwhich a bearing is provided with a magnetic fluid seal but a magnet isclamped by two pole plates and fixed to an outer ring (type disclosed inJapanese Unexamined Utility Model Registration Application PublicationNo. 1-91125). Example 1 is an example in which the bearing according tothe above-described embodiment illustrated in FIGS. 1 and 2 is used.Example 2 is an example in which the bearing according to theabove-described embodiment illustrated in FIG. 4 is used. Example 3 isan example in which the bearing according to the above-describedembodiment illustrated in FIG. 6 is used.

In Table 1, for rotational resistance, ◯ indicates that the rotationalresistance was low, and X indicates that the rotational resistance washigh. For sensory evaluation, ◯ indicates that the rotation felt smoothin the rotating operation, and X indicates that the rotation felt roughin the rotating operation. With regard to the test performed afterimmersion into salt water, the result shown was obtained by performingthe rotating operation for 1 minute while the bearing units wereimmersed in 5% salt water, and then performing the rotating operationagain after the bearing units were dried.

The bearings having the above-described sealing functions of therespective structures were also evaluated for productivity in terms ofease of assembly thereof and ease of handling thereof in the process ofattaching them to the handle shaft portions. In Table 1, for ease ofassembly,

indicates that the assembly was very easy, Δ indicates that the assemblywas rather cumbersome, and X indicates that the assembly was cumbersome.In addition, in Table 1, for ease of handling,

indicates that the bearing was very easy to handle, ◯ indicates that thebearing was easy to handle, and Δ indicates that the bearing needed tobe handled with care.

TABLE 1 Evaluation Result Rotational performance After Immersion Initialinto Salt Water Productivity Rotational Sensory Sensory Ease of Ease ofSpecifications Resistance Evaluation Evaluation Assembly HandlingRelated Art Rubber X ◯ ◯ Δ

Example 1 Packing Type Related Art Magnetic ◯ ◯ X X Δ Example 2 FluidType of Related Art Present First ◯ ◯ ◯ Δ Δ Invention 1 EmbodimentPresent Second ◯ ◯ ◯

◯ Invention 2 Embodiment Present Third ◯ ◯ ◯

Invention 3 Embodiment

With regard to the rotational performance, the rubber-packing-typebearing (Related Art Example 1) felt heavy during the operation due tolarge rotational resistance. The other magnetic-fluid-seal-type bearings(Related Art Example 2 and Examples 1 to 3) had small rotationalresistances and felt light during the operation. None of the bearingsfelt rough during the rotation in the initial stage before they wereimmersed in salt water. When the rotating operation was performed afterthe immersion into salt water, the bearing having the structure ofRelated Art Example 2 felt rough. This is because the salt waterintruded into the rolling element section through unsealed regions andformed crystals when it dried. Although the bearing of Related ArtExample 1 did not feel rough, it is expected that it will start to feelrough earlier than the bearings of Examples 1 to 3 when the experimentis repeated for a long time and the rubber packing material becomesdegraded. In the structures of Examples 1 to 3, the magnetic fluid doesnot leak out, so that the rotation feels smooth for a long period oftime.

With regard to the ease of assembly, it took time to install the rubberpacking material in the assembly of the rubber-packing-type bearing(Related Art Example 1), and it took time to assemble the bearing ofExample 1 since the process of press-fitting the magnet 12 was delicate.It took a very long time to assemble the bearing of Related Art Example2 since unitization was difficult. In contrast, the bearings of Examples2 and 3 were easy to assemble since the attraction force of a magnet isused and it is only necessary to insert a pole plate having the magnetbonded thereto from the exposed-end-surface side.

With regard to the ease of handling, the rubber-packing-type bearing(Related Art Example 1) has no problem. With regard to themagnetic-fluid-seal-type bearings, in Example 3, there was no risk thatthe magnetic fluid will be wiped off by mistake since the magnetic fluidwas disposed in an inner region. In Example 2, the magnetic fluid isdisposed in a region slightly further toward the inner region than theopening, and is therefore not easily wiped off. In Example 1 and RelatedArt Example 2, the magnetic fluid is disposed in an end region, andthere is a risk that the magnetic fluid will be wiped off by mistake.Therefore, these bearings need to be handled with care.

As is clear from the above-described test result, compared to therubber-packing-type bearing and the bearing with a magnetic fluid sealin which one of the inner and outer rings is fixed and the magneticfluid is retained by the other of the inner and outer rings according tothe related art, the structures of the present invention showed superiorresults in terms of sealing effect and productivity.

REFERENCE SIGNS LIST

-   -   1 bearing with magnetic fluid seal    -   3 inner ring    -   3 a exposed end surface    -   5 outer ring    -   5 b step    -   5 c perpendicular surface    -   5 a exposed end surface    -   7 rolling element    -   10 magnetic fluid seal    -   12 ring-shaped magnet    -   14 ring-shaped pole plate    -   15 a, 15 b magnetic fluid    -   G, G1 to G3 gap

What is claimed is:
 1. A bearing comprising: an inner ring; an outerring; a plurality of rolling elements positioned between the inner ringand the outer ring; a ring-shaped magnet positioned between the innerring and the outer ring at an axially outer side of the plurality ofrolling elements and magnetized such that magnetic poles are oriented inan axial direction; a ring-shaped plate arranged so as to be in contactwith an axially outer side surface of the ring-shaped magnet;inner-ring-side magnetic fluid retained at least between the inner ringand the ring-shaped plate and outer-ring-side magnetic fluid retained ina gap formed between an outer peripheral surface of the ring-shapedplate and an inner peripheral surface of the outer ring, wherein a stepis formed in an inner circumferential surface of the outer ring suchthat a distance between the inner and outer rings is greater at anaxially outer side than at an axially inner side, wherein thering-shaped magnet is attracted to a step surface of the step by amagnetic force, and wherein an axially outer side surface of the plateis retreated such that at least an inner-ring-side end thereof isaxially inside an axially outer side surface of the inner ring.
 2. Thebearing of claim 1 wherein the plate includes a tapered portion beingthinner radially inward.
 3. The bearing of claim 1 wherein theouter-ring-side magnetic fluid is further retained between the outerring and the ring-shaped magnet.
 4. The bearing of claim 1 wherein theouter-ring-side magnetic fluid is further retained between the outerring and an axially inner side surface of the ring-shaped magnet.
 5. Thebearing of claim 1, wherein the ring-shaped magnet is bonded to thering-shaped plate.
 6. A bearing comprising: an inner ring; an outerring; a plurality of rolling elements positioned between the inner ringand the outer ring; a ring-shaped magnet positioned between the innerring and the outer ring at an axially outer side of the plurality ofrolling elements and magnetized such that magnetic poles are oriented inan axial direction; a ring-shaped plate arranged so as to be in contactwith an axially outer side surface of the ring-shaped magnet;outer-ring-side magnetic fluid retained at least between the outer ringand the ring-shaped plate; and inner-ring-side magnetic fluid retainedin a gap formed between an inner peripheral surface of the ring-shapedplate and an outer peripheral surface of the inner ring, wherein a stepis formed in an outer circumferential surface of the inner ring suchthat a distance between the inner and outer rings is greater at anaxially outer side than at an axially inner side, wherein thering-shaped magnet is attracted to a step surface of the step by amagnetic force, and wherein an axially outer side surface of the plateis retreated such that at least an outer-ring-side end thereof isaxially inside an axially outer side surface of the outer ring.
 7. Thebearing of claim 6 wherein the plate includes a tapered portion beingthinner radially outward.
 8. The bearing of claim 6 wherein theinner-ring-side magnetic fluid is further retained between the innerring and the ring-shaped magnet.
 9. The bearing of claim 6 wherein theinner-ring-side magnetic fluid is further retained between the innerring and an axially inner side surface of the ring-shaped magnet. 10.The bearing of claim 6, wherein the ring-shaped magnet is bonded to thering-shaped plate.
 11. A bearing with a magnetic fluid seal, wherein aplurality of rolling elements are interposed between an inner ring andan outer ring, the outer ring having a step formed on an inner surfacethereof, and a ring-shaped magnet is arranged at an opening side of theinner and outer rings so as to retain magnetic fluid for sealing theplurality of rolling elements, and wherein the ring-shaped magnet ismagnetized such that magnetic poles are oriented in an axial direction,the bearing comprising: a ring-shaped plate arranged so as to be incontact with an axially outer side surface of the ring-shaped magnet;outer-ring-side magnetic fluid retained at least between the outer ringand the ring-shaped plate; and inner-ring-side magnetic fluid retainedat least between the inner ring and the ring-shaped magnet, wherein agap is formed in an inner circumferential surface of the outer ring orin an outer circumferential surface of the inner ring, wherein thering-shaped plate is press-fitted to the inner ring, and a step gap isformed between the ring-shaped magnet and the step, and wherein themagnetic fluid is disposed in the step gap.